A technique for forming a transistor by using a semiconductor over a substrate having an insulating surface has attracted attention. The transistor is applied to a wide range of semiconductor devices such as an integrated circuit and a display device. Silicon is known as a semiconductor applicable to a transistor.
As silicon which is used as a semiconductor of a transistor, either amorphous silicon or polycrystalline silicon is used depending on the purpose. For example, in the case of a transistor included in a large display device, it is preferable to use amorphous silicon, which can be used to form a film on a large substrate with the established technique. In the case of a transistor included in a high-performance display device where driver circuits are formed over the same substrate, it is preferred to use polycrystalline silicon, which can form a transistor having high field-effect mobility. It is known that polycrystalline silicon can be formed as a result of heat treatment at high temperatures or laser light treatment on amorphous silicon.
In recent years, transistors including oxide semiconductors (typified by an In—Ga—Zn oxide) have been actively developed.
Oxide semiconductors have been researched since early times. In 1988, it was disclosed to use an In—Ga—Zn oxide crystal for a semiconductor element (see Patent Document 1). In 1995, a transistor including an oxide semiconductor was invented, and its electrical characteristics were disclosed (see Patent Document 2).
In 2014, it was reported that a transistor including a crystalline In—Ga—Zn oxide has more excellent electrical characteristics and higher reliability than a transistor including an amorphous In—Ga—Zn oxide (see Non-Patent Document 1). Non-Patent Document 1 reports that a crystal boundary is not clearly observed in an In—Ga—Zn oxide including a c-axis aligned crystalline oxide semiconductor (CAAC-OS).